The present invention relates generally to a projection optical system in an exposure apparatus, and more particularly to a projection optical system in an exposure apparatus used to manufacture a semiconductor integrated circuit and a liquid crystal display.
The exposure apparatus, such as a stepper and a scanner, used to manufacture a semiconductor device, such as an IC and LSI, is demanded to have an improved resolving power as the fine processing to the semiconductor device advances.
In general, the Rayleigh's formula gives the resolving power of the exposure apparatus, where R is a resolvable critical dimension, k1 is a constant determined by the photosensitive agent (resist) and illumination condition, λ is an exposure wavelength, and NA is a numerical aperture:R=k1·λ/NA  (1)
NA is defined as follows, where n is a refractive index of an image side, and θ is an angle between the optical axis and the marginal ray:NA=n·sin θ  (2)
The depth of focus (“DOF”) is expressed as follows, where k2 is a constant:DOF=k2·n·λ/(NA2)  (3)
Therefore, according to Equation (1), use of a shorter exposure wavelength and a higher NA of the projection optical system are effective to the improved resolving power of the exposure apparatus.
The use of a shorter exposure wavelength has been promoted by adopting the ultra high-pressure mercury lamp (having a wavelength of 365 nm), a KrF excimer laser (having a wavelength of 248 nm), an ArF excimer laser (having a wavelength of 193 nm), etc. As for the NA of the projection optical system, an exposure apparatus equipped with a projection optical system having an NA of 0.85 is reduced to practice. A projection optical system exceeding an NA of 1.0 is being studied by applying the immersion technology that is used for the microscope field to the semiconductor exposure apparatus.
However, various technological problems arise as the high NA advances, such as 1) difficulties of corrections of various aberrations due to the high NA, 2) increased cost disadvantages caused by a large projection optical system, 3) manufacturing difficulties of a large aperture lens for the large projection optical system, 4) design and manufacturing difficulties of an antireflection coating applied to a lens, 5) remarkable influence of the polarization in imaging, and 6) a decreased focus margin due to the reduced DOF in inverse proportion to square NA.
A description will be given of a typical projection optical system for a conventional exposure apparatus. FIG. 15 shows a structure of the conventional projection optical system. Tables 7 and 8 indicate a radius of curvature, a surface interval, an effective diameter, and an aspheric coefficient for each surface.
The projection optical system has a specification of an NA of 1.1 (immersion), a light source of the ArF excimer laser (having a wavelength of 193 nm), and the maximum object point of 53.4 mm. Calculations assume that synthetic quartz (SiO2) has a refractive index of 1.5603, calcium fluoride (CaF2) has a refractive index of 1.5014, and water has a refractive index of 1.4367. The values of these refractive indexes are common to the embodiments of the present invention, which will be described later.
Unlike an illumination optical system, the projection optical system in the exposure apparatus and an objective lens in a microscope are required to have performance compatible with the diffraction limits. Generally speaking, the projection optical system and the like are required to have a wavefront aberration of 0.07λRMS or smaller as a permissible residue aberration, which is referred to as a Marechal's criterion.
The more recent semiconductor exposure apparatus is required to have a smaller wavelength aberration. The projection optical system shown in FIG. 15 has a value of 0.005λRMS throughout the entire screen area, and has received an excellent aberrational correction.
It is understood from Table 7 that the largest lens has an effective diameter of Φ350 mm or greater in the projection optical system. The block weight in the rightmost column in Table 7 denotes the weight of the cylindrical glass block that circumscribes each lens, and of materials necessary to manufacture the lens. The calculation assumes a margin from the effective diameter to the lens outer diameter to be 5 mm, a margin from the effective diameter of the concave surface to the outer diameter of the concave surface to be 2 mm, and a polishing margin to be 1 mm. These margins are common to the following embodiments.
Japanese Patent Application, Publication No. 2004-22708 discloses, at paragraphs 0038 to 0041, and FIG. 15 etc., an imaging optical system that serves as an illumination optical system for an exposure apparatus, forms an intermediate image, and has a reduced lens diameter.
It is understood from Table 7 that about 220 kg of glass material is necessary to manufacture the projection optical system. Therefore, when expensive synthetic quartz for the ArF excimer laser is used to manufacture the projection optical system, the whole exposure apparatus becomes expensive due to the material cost of the optical element.
In addition, the glass material used for the semiconductor exposure apparatus should maintain the homogeneity extremely high and the birefringence extremely low, and it is technically difficult to satisfy the specification in the glass material having a large diameter.
Moreover, a diameter of the above conventional projection optical system exponentially increases, as the high NA scheme advances. FIG. 6 shows a relationship between the NA and the lens diameter in various types of projection optical systems, as disclosed in SPIE (The International Society for Optical Engineering), February of 2003. According to FIG. 6, the lens diameter increases nonlinearly in the projection optical system as the NA increases. The conventional projection optical system shown in FIG. 15 is an immersion dioptric optical system that uses an aspheric lens, and the lens diameter suddenly increases as the NA exceeds 1.05. The glass material cost and the manufacturing difficulty increasingly will rise in designing and manufacturing a future projection optical system having a higher NA that meets the demands for the improved resolving power.
According to FIG. 6, a catadioptric projection optical system has a smaller diameter than a dioptric projection optical system. However, the catadioptric system should use a mirror (reflective element) that has a higher manufacturing sensitivity than a lens (refractive element), and poses higher design and manufacturing difficulties than the dioptric system. Therefore, instead of using the catadioptric system, the dioptric system preferably forms a projection optical system having a high NA.